Effects of activity, hemorrhage, and dehydration on plasma catecholamine levels in the marine toad (Bufo marinus)

Resting plasma epinephrine and norepinephrine levels were 13.1 and 2.1 nmol liter-1 for the marine toad (Bufo marinus). Plasma catecholamine levels increased during enforced activity by five- to sixfold. Marine toads are remarkably tolerant of graded hemorrhagic loss of blood (over 10% mass loss). Plasma catecholamine levels did not increase at moderate blood loss, but increased substantially when cardiovascular variables (blood pressure, blood flow) were compromised and peripheral resistance was increased. Plasma catecholamine levels did not increase with dehydrational mass loss until a 15-20% loss of mass. The increase in plasma catecholamine concentration was correlated with an increase in vivo vascular resistance. Vascular resistance measured in vitro was unaltered at physiological catecholamine concentrations, although systemic resistance increased at pharmacological concentrations. The lack of effects of adrenalectomy on plasma catecholamine levels suggests that nerve terminal release, rather than adrenal secretion, may be the primary source of circulating catecholamines. We therefore suggest that circulating catecholamine levels are not an important endocrinological mechanism for defense of activity blood pressure, at least until it is compromised to the resting value.